by S. R. Mulrune
Elle is a new agent in an inter-universal company. They save people on alternate Earths, but before she can become a hero, she has to come to terms with going to mirror Earths. She can't quite grasp the full meaning of alternate realities, until one she's sure of shatters her assumptions.
This is a short story in the Lone Wolves series, which is part of the Worldshift Saga universe.